Method for fabricating vertical channel type nonvolatile memory device

ABSTRACT

A method for fabricating a vertical channel type nonvolatile memory device includes: stacking a plurality of interlayer insulating layers and a plurality of gate electrode conductive layers alternately over a substrate; etching the interlayer insulating layers and the gate electrode conductive layers to form a channel trench exposing the substrate; forming an undoped first channel layer over the resulting structure including the channel trench; doping the first channel layer with impurities through a plasma doping process; and filling the channel trench with a second channel layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/493,439 filed on Jun. 29, 2009 now U.S. Pat. No. 8,048,743, whichclaims priority of Korean Patent Application No. 10-2009-0052159, filedon Jun. 12, 2009. The disclosure of each of the foregoing applicationsis incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method for fabricating a nonvolatilememory device, and more particularly, to a method for fabricating athree-dimensional (3D) nonvolatile memory device.

Nonvolatile memory devices retain stored data even when power isinterrupted. However, two-dimensional (2D) memory devices fabricated ina single layer on a silicon substrate have limitations in improvingintegration density. Therefore, 3D nonvolatile memory devices withmemory cells stacked vertically from a silicon substrate are desirable.

Hereinafter, the structure and limitation of a conventional 3Dnonvolatile memory device will be described in detail with reference toFIG. 1.

FIG. 1 is a cross-sectional view of a conventional 3D nonvolatile memorydevice. Specifically, FIG. 1 is a cross-sectional view of a nonvolatilememory device in which strings are vertically arranged over a substrate.For the sake of convenience, a description about a process of forming alower selection transistor and an upper selection transistor is omitted.

Referring to FIG. 1, a plurality of interlayer insulating layers 11 anda plurality of gate electrode conductive layers 12 are alternatelyformed on a substrate 10 having required lower structures such as sourcelines and lower select transistors that are formed, for example, belowthe interlayer insulating layers 11 and the plurality of gate electrodeconductive layers 12. Thereafter, the interlayer insulating layers 11and the gate electrode conductive layers 12 are selectively etched toform a channel trench exposing the surface of the substrate 10.

A charge blocking layer, a charge trapping layer, and a tunnelinsulating layer are sequentially formed over the resulting structureincluding the channel trench. Thereafter, an etch-back process isperformed to expose the surface of the substrate 10. For illustrationpurposes, the charge blocking layer, the charge trapping layer, and thetunnel insulating layer are shown as one layer denoted by a referencenumeral 13.

The channel trench is filled with a channel layer to form a channel 14protruding vertically from the substrate 10. Herein, the channel layermay be formed by growing a monocrystalline silicon layer through anepitaxial growth process, or may be formed by depositing a polysiliconlayer through a chemical vapor deposition (CVD) process.

Consequently, a plurality of stacked memory cells are formed along thechannel 14 protruding vertically from the substrate 10, and the memorycells are connected in series between a lower select transistor (notshown) and an upper select transistor (not shown) to constitute onestring.

However, the foregoing conventional method has limitations incontrolling the doping concentration of the channel 14.

In general, for fabrication of a nonvolatile memory device, the dopingconcentration of the channel 14 is controlled to control the thresholdvoltage of the memory cells. For example, the threshold voltage iscontrolled by doping the channel layer with n-type impurities at a lowconcentration. In the case of a conventional planar nonvolatile memorydevice, a channel layer is formed and the channel layer is doped withimpurities through an ion implantation process, thereby forming achannel with a low doping concentration.

However, in the case of the vertical channel type nonvolatile memorydevice, because the channel trench is filled with the channel layer toform the channel 14, doping by ion implantation is difficult toimplement. Also, even when the channel layer is formed through a dopingprocess, it is not easy to implement a doping concentration of less than1E19 atoms/cm³. That is, according to the conventional method, it isimpossible to form a vertical channel with a low doping concentrationfor fabrication of a 3D nonvolatile memory device.

For illustration purposes, limitations that occur in forming a channelof a memory cell have been explained. However, such limitations may alsooccur in forming a lower select transistor or an upper selecttransistor.

SUMMARY OF THE INVENTION

An embodiment of the present invention is directed to provide a methodfor fabricating a 3D nonvolatile memory device, which can easily controlthe doping concentration of a channel.

In accordance with an aspect of the present invention, there is provideda method for fabricating a vertical channel type nonvolatile memorydevice, the method including: stacking a plurality of interlayerinsulating layers and a plurality of gate electrode conductive layersalternately over a substrate; etching the interlayer insulating layersand the gate electrode conductive layers to form a channel trenchexposing the substrate; forming an undoped first channel layer over theresulting structure including the channel trench; doping the firstchannel layer with impurities through a plasma doping process; andfilling the channel trench with a second channel layer.

In accordance with another aspect of the present invention, there isprovided a method for fabricating a vertical channel type nonvolatilememory device, the method including: stacking a plurality of interlayerinsulating layers and a plurality of gate electrode conductive layersalternately over a substrate; etching the interlayer insulating layersand the gate electrode conductive layers to form a channel trenchexposing the substrate; and alternately forming first and second channellayers with different doping concentrations in the channel trench.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional 3D nonvolatile memorydevice.

FIGS. 2A to 2C are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anembodiment of the present invention.

FIGS. 3A to 3D are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anotherembodiment of the present invention.

FIGS. 4A to 4D are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anotherembodiment of the present invention.

FIGS. 5A and 5B are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anotherembodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Other objects and advantages of the present invention can be understoodby the following description, and become apparent with reference to theembodiments of the present invention.

Referring to the drawings, the illustrated thickness of layers andregions are exemplary and may not be exact. When a first layer isreferred to as being “on” a second layer or “on” a substrate, it couldmean that the first layer is formed directly on the second layer or thesubstrate, or it could also mean that a third layer may exist betweenthe first layer and the second layer or the substrate. Furthermore, thesame or like reference numerals represent the same or like components,even if they appear in different embodiments or drawings of the presentinvention.

FIGS. 2A to 2C are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anembodiment of the present invention.

Referring to FIG. 2A, a plurality of interlayer insulating layers 21 anda plurality of gate electrode conductive layers 22 are alternatelyformed on a substrate 20.

Herein, the interlayer insulating layers 21 serve to isolate a pluralityof memory cells from each other. The interlayer insulating layer 21 mayinclude an oxide layer. Also, the gate electrode conductive layer 22 mayinclude a polysilicon layer.

Also, the interlayer insulating layer 21 and the gate electrodeconductive layer 22 may be formed repeatedly according to the number ofmemory cells to be stacked on the substrate 20. For illustrationpurposes, the present embodiment illustrates the case of stacking twomemory cells.

The interlayer insulating layers 21 and the gate electrode conductivelayers 22 are etched to form a channel trench exposing the surface ofthe substrate 20.

A charge blocking layer, a charge trapping layer, and a tunnelinsulating layer are sequentially formed over the resulting structureincluding the channel trench, and an etch-back process is performed toexpose the surface of the substrate 20. For illustration purposes, thecharge blocking layer, the charge trapping layer, and the tunnelinsulating layer are illustrated as one layer denoted by a referencenumeral 23.

An undoped first channel layer 24 is formed over the resulting structureincluding the charge blocking layer, the charge trapping layer, and thetunnel insulating layer. Herein, the first channel layer 24 is formedsuch that a center region of the channel trench is hollow.

A plasma doping process is performed to dope the first channel layer 24with impurities. For example, a first channel layer 24 may include anundoped polysilicon layer, and the first channel layer 24 may be dopedwith n-type impurities.

Through the plasma doping process, the first channel layer 24 is dopedwith impurities to a certain thickness from the surface of the firstchannel layer 24. Herein, the doping concentration is highest at thesurface of the first channel layer 24, and decreases with an increase indepth from the surface of the first channel layer 24.

Referring to FIG. 2B, a heavily-doped region of the first channel layer24 is etched to a predetermined thickness. That is, the first channellayer 24 is etched to a predetermined thickness from the surface of thefirst channel layer 24 to remove the high-concentration impurities ofthe first channel layer 24, thereby leaving only the low-concentrationimpurities of the first channel layer 24A. The first channel layer 24etched to the predetermined thickness is denoted by a reference numeral24A.

A second channel layer 25 is formed on the resulting structure to fillthe channel trench. The second channel layer 25 may include an undopedpolysilicon layer.

Referring to FIG. 2C, a planarization process is performed to expose thesurface of the interlayer insulating layer 21, thereby forming a channelCH including a first channel layer 24B and a second channel layer 25A.

A thermal treatment process is performed to diffuse the doped impuritiesof the first channel layer 24B into the second channel layer 25A,thereby forming a channel CH with a low doping concentration.

FIGS. 3A to 3D are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anotherembodiment of the present invention. A description of an overlap withthe embodiment of FIGS. 2A to 2C will be omitted.

Referring to FIG. 3A, a plurality of interlayer insulating layers 31 anda plurality of gate electrode conductive layers 32 are alternatelyformed on a substrate 30. The interlayer insulating layers 31 and thegate electrode conductive layers 32 are etched to form a channel trenchexposing the surface of the substrate 30.

A charge blocking layer, a charge trapping layer, and a tunnelinsulating layer are sequentially formed over the resulting structureincluding the channel trench, and an etch-back process is performed toexpose the surface of the substrate 30. For illustration purposes, thecharge blocking layer, the charge trapping layer, and the tunnelinsulating layer are illustrated as one layer denoted by a referencenumeral 33.

A first channel layer 34 is formed over the resulting structureincluding the charge blocking layer, the charge trapping layer, and thetunnel insulating layer 33. For example, the first channel layer 34 mayinclude an undoped polysilicon layer.

Referring to FIG. 3B, a buffer layer 35 is formed on the first channellayer 34. Herein, the buffer layer 35 serves to prevent the firstchannel layer 34 from being directly doped with impurities in thesubsequent process. The buffer layer 35 may include an oxide layer or anitride layer. For example, if the buffer layer 35 is an oxide layer, anoxide layer may be deposited on the first channel layer 34 or thesurface of the first channel layer 34 may be oxidized to a predeterminedthickness through an oxidation process to form the buffer layer 35.

A plasma doping process is performed to dope the first channel layer 34with impurities. The buffer layer 35 on the first channel layer 34 isdoped with impurities, and then the first channel layer 34 is doped withimpurities. The buffer layer 35 can prevent the first channel layer 34from being directly doped with impurities. Thus, the buffer layer 35 isdoped at a relatively high doping concentration, and the first channellayer 34 is doped at a relatively low doping concentration.

The thickness of the buffer layer 35 may be controlled to control thedoping concentration of the first channel layer 34. For example, thedoping concentration of the first channel layer 34 may be reduced byincreasing the thickness of the buffer layer 35.

Referring to FIG. 3C, the buffer layer 35 is removed to remove theheavily-doped region. The buffer layer 35 may be removed through a wetetching process.

A second channel layer 36 is formed over the resulting structure withoutthe buffer layer 35. The second channel layer 36 may include an undopedpolysilicon layer.

Referring to FIG. 3D, a planarization process is performed to expose thesurface of the interlayer insulating layer 31, thereby forming a channelCH including a first channel layer 34A and a second channel layer 36A.

A thermal treatment process is performed to diffuse the doped impuritiesof the first channel layer 34A into the second channel layer 36A,thereby forming a channel CH with a low doping concentration.

FIGS. 4A to 4D are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anotherembodiment of the present invention. A detailed description of theprocess that overlaps with the description of the foregoing embodimentswill be omitted.

Referring to FIG. 4A, a plurality of interlayer insulating layers 41 anda plurality of gate electrode conductive layers 42 are alternatelyformed on a substrate 40. The interlayer insulating layers 41 and thegate electrode conductive layers 42 are etched to form a channel trenchexposing the surface of the substrate 40.

A charge blocking layer, a charge trapping layer, and a tunnelinsulating layer are sequentially formed over the resulting structureincluding the channel trench, and an etch-back process is performed toexpose the surface of the substrate 40. For illustration purposes, thecharge blocking layer, the charge trapping layer, and the tunnelinsulating layer are illustrated as one layer denoted by a referencenumeral 43.

A first channel layer 44 is formed over the resulting structureincluding the charge blocking layer, the charge trapping layer, and thetunnel insulating layer. The first channel layer 44 may include anundoped polysilicon layer.

A plasma doping process is performed to dope the first channel layer 44with impurities. A first channel layer 44 may include an undopedpolysilicon layer, and the first channel layer 44 may be doped withn-type impurities. The doping concentration is highest at the surface ofthe first channel layer 44, and decreases with an increase in depth fromthe surface of the first channel layer 44.

Referring to FIG. 4B, a buffer layer 45 is formed on the first channellayer 44. The buffer layer 45 serves to remove the heavily-doped regionof the first channel layer 44. The buffer layer 45 may include an oxidelayer or a nitride layer.

For example, if the buffer layer 45 is an oxide layer, the surface ofthe first channel layer 44 may be oxidized to a predetermined thicknessthrough an oxidation process to form an oxide layer. At this point, thesurface of the first channel layer 44 with a high doping concentrationis oxidized to form the oxide layer, i.e., the buffer layer 45.

The buffer layer 45 formed through the oxidation process is removed inthe subsequent process, thereby removing the heavily-doped region. Thus,the remaining doping concentration of the first channel layer 44 may becontrolled by controlling the oxidation level of the first channel layer44, i.e., the thickness of the buffer layer 45. For example, theremaining doping concentration of the first channel layer 44 may bereduced by increasing the oxidation thickness of the first channel layer44, i.e., by increasing the thickness of the buffer layer 45.

Referring to FIG. 4C, the buffer layer 45 is removed. The buffer layer45 may be removed through a wet etching process.

In this way, the impurities of the surface of the first channel layer 44heavily doped through the plasma doping process can be removed byremoving the buffer layer 45 formed by oxidizing the surface of thefirst channel layer 44. Accordingly, the heavily-doped region is removedand only the lightly-doped region remains in a first channel layer 44A.

A second channel layer 46 is formed over the resulting structure withoutthe buffer layer 45. The second channel layer 46 may include an undopedpolysilicon layer.

Referring to FIG. 4D, a planarization process is performed to expose thesurface of the interlayer insulating layer 41, thereby forming a channelCH including a first channel layer 44A and a second channel layer 46A.

A thermal treatment process is performed to diffuse the doped impuritiesof the first channel layer 44B into the second channel layer 46A,thereby forming a channel CH with a low doping concentration.

FIGS. 5A and 5B are cross-sectional views illustrating a process forfabricating a 3D nonvolatile memory device in accordance with anotherembodiment of the present invention. A detailed description thatoverlaps with the description of the foregoing embodiments of thepresent invention will be omitted.

Referring to FIG. 5A, a plurality of interlayer insulating layers 51 anda plurality of gate electrode conductive layers 52 are alternatelyformed on a substrate 50. The interlayer insulating layers 51 and thegate electrode conductive layers 52 are etched to form a channel trenchexposing the surface of the substrate 50.

A charge blocking layer, a charge trapping layer, and a tunnelinsulating layer 53 are sequentially formed over the resulting structureincluding the channel trench, and an etch-back process is performed toexpose the surface of the substrate 50. For illustration purposes, thecharge blocking layer, the charge trapping layer, and the tunnelinsulating layer are illustrated as one layer denoted by a referencenumeral 53.

A first channel layer and a second channel layer are alternately formedin the channel trench. The first channel layer and the second channellayer have different doping concentrations. The first channel layer hasa relative low doping concentration and the second channel layer has arelatively high doping concentration. Hereinafter, a description isprovided for an exemplary case where the first channel layer 54 isundoped and the second channel layer 55 is doped.

A first channel layer 54 is formed over the resulting structureincluding the charge blocking layer, the charge trapping layer, and thetunnel insulating layer.

A doped second channel layer 55 is formed on the resulting structureincluding the first channel layer 54. The second channel layer 55 may beformed through a doping process. More specifically, the second channellayer 55 may include a polysilicon layer doped with n-type impurities.

Referring to FIG. 5B, a thermal treatment process is performed todiffuse the doped impurities of the second channel layer 55 into thefirst channel layer 54.

A planarization process is performed to expose the surface of theinterlayer insulating layer 51, thereby forming a lightly-doped channelCH including a first channel layer 54A and a second channel layer 55A.

Even if the present embodiment has been described an exemplary case offorming the channel including the first channel layer 54 and the secondchannel layer 55, a plurality of channel layers may also be repeatedlyformed to form a multi-stack channel.

As described above, a first channel layer may be doped with impuritiesthrough a plasma doping process, and the heavily-doped region may beetched to a predetermined thickness, thereby making it possible to forma lightly-doped channel. In particular, a buffer layer is formed on thefirst channel layer and removed in the subsequent process, therebymaking it possible to easily remove the heavily-doped region.

Also, the first and second channel layers are alternately formed withdifferent doping concentrations, and a thermal treatment process isperformed on the resulting structure, thereby making it possible toeasily form a lightly-doped channel.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A method for fabricating a vertical channel type nonvolatile memorydevice, comprising: stacking a plurality of interlayer insulating layersand a plurality of conductive layers alternately over a substrate;etching the interlayer insulating layers and the conductive layers toform a channel trench exposing the substrate; forming an undoped firstchannel layer over a structure including the channel trench; forming abuffer layer over the undoped first channel layer; performing a dopingprocess onto the stacked structure including the buffer layer and theundoped first channel layer with impurities to form a doped firstchannel layer and a doped buffer layer, wherein the doped first channellayer has a lower concentration of impurities than a concentration ofimpurities in the doped buffer layer; removing the doped buffer layerafter performing of the doping process; and filling the channel trenchwith a second channel layer.
 2. The method of claim 1, wherein thedoping concentration of the first channel layer is controlled bycontrolling a thickness of the buffer layer formed over the undopedfirst channel layer.
 3. The method of claim 1, wherein the buffer layercomprises an oxide layer or a nitride layer.
 4. The method of claim 1,further comprising: performing a thermal treatment process after fillingthe channel trench with the second channel layer.
 5. The method of claim4, wherein the performing of a thermal treatment process includesdiffusing the doped impurities of the doped first channel layer into thesecond channel layer.
 6. The method of claim 1, further comprising:forming a charge blocking layer, a charge trap layer, and a tunnelinsulating layer over the structure including the channel trench beforethe forming of the undoped first channel layer.
 7. The method of claim1, wherein the conductive layer is a gate electrode.
 8. The method ofclaim 1, further comprising: performing a thermal treatment processafter filling the channel trench with the second channel layer.
 9. Themethod of claim 8, wherein the performing of a thermal treatment processincludes diffusing the doped impurities of the doped first channel layerinto the second channel layer.
 10. The method of claim 1, wherein thesecond channel layer is comprised of undoped poly silicon.